1. Field of the Invention
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine installing an NOx (nitrogen oxides) absorbent in an exhaust conduit of the engine, and particularly relates to an exhaust gas purification apparatus capable of recovering an NOx absorbent poisoned by sulfur oxides (SOx).
2. Description of the Related Art
Internal combustion engines capable of fuel combustion at lean air-fuel ratios (lean burn engines) have been noted as engines which can both improve fuel economy and suppress the exhaust of CO.sub.2. However, since a conventional catalyst (three-way catalyst) cannot purify NOx in exhaust gas from an engine operated at lean air-fuel ratios (that is, in exhaust gas including excess oxygen), catalysts or systems that can suppress the exhaust of NOx into the atmosphere even at lean air-fuel ratios are desired.
For example, Japanese Patent Application HEI 4-73264 filed Feb. 25, 1992 proposes an exhaust gas purification apparatus wherein an NOx absorbent including lanthanum (La), which is one kind of rare-earth, and platinum (Pt) is installed in an engine exhaust conduit. In the apparatus, NOx is absorbed by the La during an engine operation at lean air-fuel ratios, and the NOx absorbed by the La is released from the La at an engine operation at a stoichiometric or rich air-fuel ratio and is decomposed (or reduced) into N.sub.2.
However, it has been found in tests executed for developing the present invention that if a high temperature and excess oxygen exhaust gas condition continues for a relatively long time period, the NOx absorption rate of the NOx absorbent decreases.
One reason for such degradation of the NOx absorbent is presumed to be that SOx (for example, SO.sub.2 which is produced from sulfur contained in fuel) is oxidized to sulfate ion (SO.sub.4.sup.2-), which is diffused into and is absorbed by the NOx absorbent to form an obstinate sulfate which obstructs the absorption of NOx by the NOx absorbent.